The present invention relates to integrated bus drivers.
Background: Bus Interface Device
Integrated bus interface devices must be capable of sinking or sourcing high output current when the output switches. A current in the range from 50-100 mA is typical. The high current is necessary to achieve fast propagation delay times (typically driving a 75-ohm transmission line). After the output has reached the stable steady state, only a small part of the switching current, or even no current is needed.
In BiCMOS chips, bipolar NPN transistors were traditionally used to deliver the high sink current that was needed for bus driver devices. However, while bipolar output drivers provide fast high-current output switching, they have the disadvantage of a large static power consumption.
Background: Power-On-Demand Technology
In present day computer systems, the buses spend 80% or more of their time in the stable, inactive (low or high) state. Modern bus interface ICs take this into account. The circuits are designed for self-control of internal power consumption based upon what is needed to drive the output load. Power-On-Demand ("POD") circuits reduce power drain. The first circuits employing this feature appeared in 1992. However, a major limitation of these earlier designs is that the output clamping voltage is dependent on the output current. If current through the output transistor increases, the base-emitter voltage Vbe also increases, due in part to the emitter resistance of the output transistor. See Applicant's prior German application (#De 195 47 754 C1, Control Circuit for a BiCMOS Bus Driver), which is hereby incorporated by reference.
Fast Bus Driver with Reduced Standby Power Consumption
The present application discloses an innovative technique for controlling power consumption in a BiCMOS bus driver circuit. A bipolar output driver is used with a MOS base drive circuit which includes a feedback relationship: as output current rises, the base current also rises. The base drive circuit reacts quickly to changes in the load impedance. The bipolar output driver still provides fast switching, and high peak current when needed, but the base drive circuit limits the static base current to a small value except when more is needed. The preferred feedback circuit configuration is nearly temperature and process compensated, and therefore easy to implement in different processes.
Thus the disclosed innovations reduce wasted power consumption. Moreover, the disclosed innovations can be used advantageously for all bus driver devices that are processed in BiCMOS technology. Another advantage is that the base current of the lower output transistor is controlled such that the output voltage stays constant regardless of how large or small the output sink current is. Another advantage is that the power consumption of the circuit is strongly related to the output sink current. Another advantage is that the technique works in the various power supply regimes of 2.5V, 3.3V, or 5.0V.